There are several hundred underground coal fires currently burning out out of control in the United States, the number of such fires remaining relatively constant through the years due to accidental starting of new fires about as often as old fires are terminated. Frequently these fires are in abandoned coal mines. In other cases the fires began at an outcrop of virgin coal, then propagated underground. In some cases extensive fire termination procedures have been applied at a site to the point where it was concluded that positive fire termination had occurred, only to discover years later that the fire is still active. Underground coal fires are difficult to terminate positively.
One common problem is present in all underground coal fires: localized hot spots associated with the fire. The overburden and underburden of a coal seam typically are poor conductors of heat, as is the coal itself. Thus localized hot spots can remain at a temperature conducive to reignition for long periods of time measured in decades. Should a source of oxygen become available to the localized hot spot, propagation of the fire can resume. Since it is virtually impossible to assure that future source of oxygen will not become available, steps should be taken during fire termination to eliminate localized hot spots as a standard procedure.
Specialty formulations of foaming mud cement have been demonstrated to be effective in terminating coal fires. This material may be described as a mixture of cement and/or lime, soil as the aggregate, and water, with the foaming action provided by carbon dioxide. The practical mixtures of foaming mud cement expand in the range of 1.0 to about three times the original volume, then set into cellular concrete with compressive strengths suitable to replace the missing coal. Freshly mixed foaming mud cement is applied to burning coal, quickly forming a crust over live coals, thereby snuffing the fire. Upon initial contact of foaming mud cement with live coals, water normally used for hydration of the mixture is flashed to steam and carbon dioxide is released through expansion, both actions serving to lower the temperature of the localized hot spot. Continuing application of foaming mud cement forms a crust, then serves as a backfill for the burn cavity. Preferably the complete cavity is filled to terminate potentially serious problems of subsidence.
In the use of foaming mud cement as described in the foregoing paragraphs, there is a transition zone between the snuffed coal and the set cellular concrete. The transition zone is approximately one to three inches thick, depending on whether the coal fire is in a smoldering state or in active flaming. Within the zone are char, ashes, unhydrated cement and/or lime, and soil or other aggregate. Outside the transition zone, on one side is the remnant coal at elevated temperature and on the other side is the foaming mud cement which begins setting into cellular concrete within a few hours.
Ordinary concrete is a relatively poor conductor of heat. Cellular concrete is a much better insulator against flow of heat that it is as a conductor of heat. Thus cellular concrete as described in the foregoing paragraphs is not suitable to assure timely elimination of localized hot spots.